#include<limits.h>
//  Definition for a binary tree node.
  struct TreeNode {
      int val;
      TreeNode *left;
      TreeNode *right;
      TreeNode() : val(0), left(nullptr), right(nullptr) {}
      TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
      TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
  };
 

class Solution {
    long prev = LONG_MIN;
public:
    bool isValidBST(TreeNode* root)
    {
        if (root == nullptr) return true;

        bool left = isValidBST(root->left);
        bool cur = false;
        if (root->val > prev)
            cur = true;
        prev = root->val;

        bool right = isValidBST(root->right);
        return left && right && cur;
    }
};